Electromagnetic Induction

The Phenomenon

Electromagnetic induction: when the magnetic flux changes, a current appears in a closed circuit (Faraday, 1831).

Laws

• Magnetic flux: Φ = B·S·cosα, the unit is the weber (Wb). Here α is the angle between the field and the normal to the circuit.
• Faraday's law: ε = −ΔΦ/Δt.
• Lenz's law: the induced current opposes the cause that produced it.
• Self-induction: ε = −L·ΔI/Δt, where L is the inductance, the unit is the henry (H).

Quantity	Unit
Magnetic induction B	tesla (T)
Magnetic flux Φ	weber (Wb)
Inductance L	henry (H)

Applications

• A transformer changes an alternating voltage: U₁/U₂ = n₁/n₂.
• A generator converts mechanical energy into electrical energy.

Examples

• If the magnetic flux changes by 1.5 Wb in 0.5 s, then ε = 1,5/0,5 = 3 V.
• If n₁ = 200, n₂ = 50, U₁ = 220 V, then U₂ = 220·50/200 = 55 V.

The EMF depends on the rate of change of the flux, not on the flux itself. If the flux is large but does not change, the EMF is zero.